I just checked and the French have it too, I can see how it has slowly made its way into English, through immigration and especially the Internet.
A live language is always changing, resistance is futile. :)
This is close to being a contradiction in terms. The purpose of words is to communicate, and if a word is being successfully used to communicate -- which clearly it is -- what exactly does it mean to say that isn't "considered a real word"?
Also, "Considered" by whom? The dictionary? Dictionaries are descriptivist -- they record usage, they don't prescribe it [0]. If it isn't in the dictionary yet, that's only because the usage hasn't been used widely and/or for long enough that it meets the inclusion criteria [1].
(As it happens, "performant" is at the stage where it is starting to pass those thresholds, and is now in the OED [2])
[0] http://englishplus.com/news/news1100.htm
[1] http://public.oed.com/about/frequently-asked-questions/#qual...
Strictly speaking, I don't believe your [2] is the OED. It's "Oxford Living Dictionaries", which I assume tries to be more current/dynamic, but might be regarded as less authoritative.
Interestingly, the word "performant" is in the OED itself, with the earliest citation being from 1809 -- but it is listed only as a noun, meaning "A person who performs a duty, ceremony, etc., a performer", not the adjectival usage under discussion here.
http://www.oed.com/view/Entry/262085?redirectedFrom=performa... (requires login)
https://dictionary.cambridge.org/dictionary/english/performa...